1. Field of the Invention
The present invention relates to an acoustic apparatus of an audio/video system and, more particularly, to an apparatus for automatically adjusting a common mode rejection ratio of a balance circuit to improve tone quality.
2. Description of the Related Art
Typically, achieving high tone quality is an important concern in audio/video systems. Further, an important factor in achieving high tone quality is a signal-to-noise (S/N) ratio. The S/N ratio is adjusted by using a balance circuit. FIG. 1 shows a conventional balance circuit. A signal Vi1 received through a left channel Lch is applied to a non-inverting input terminal of an operational amplifier 10 used as a mixer. A signal Vi2 received through a right channel Rch is applied to an inverting input terminal of the operational amplifier 10 which is used as a mixer. The operational amplifier (mixer) 10 can obtain a high S/N ratio without being greatly influenced by external noise when signals supplied to inverting and non-inverting input terminals of the mixer 10 have symmetrical levels. In order to supply symmetrical signal levels to the inverting and non-inverting input terminals of the mixer 10, the output of the mixer 10 should be maintained at "0" by adjusting the input resistances of the input terminals of the mixer 10. Output values Vo1 and Vo2 of the non-inverting and inverting input terminals of the mixer 10 can be obtained by the following equations (1) and (2), respectively: ##EQU1## When the output of the mixer 10 is "0", the following conditions should be satisfied as derived from the equations (1) and (2): Vi1=Vi2, and R1=(R2+VR)=R3=R4.
Consequently, a high S/N ratio can be obtained by appropriately adjusting the resistance of a variable resistor VR. However, in order to adjust the S/N ratio using such a balance circuit, a user must manually adjust an internal variable resistance. As a result, it is difficult to minutely adjust the variable resistance when a signal is attenuated to -80 dB or more. Moreover, even if the internal variable resistance of the balance circuit is set, the set variable resistance may vary because of an external shock.